Oracle Sql Statement for unique timestamp for each row

The following UPDATE statement will guarantee that each row has a unique MY_TIMESTAMP value, by increasing the milliseconds by the rownum value.

EDIT:
After Alessandro Rossi pointed out that there could be duplicate values, the following query has been modified to use SYSTIMESTAMP for the update.

  UPDATE ITEM_HISTORY 
  SET my_timestamp = SYSTIMESTAMP + NUMTODSINTERVAL(rownum/1000, 'SECOND');

However, it is recommended that you use an alternative strategy, such as adding another column to store the key value.